I would check your local pet stores for bags. Here is some advice for shipping:
Double the bag and make sure the bag is 1 part water and 2 parts air. Also put an Almond leaf in the bag with the betta to reduce stress. Keep in mind the temps of where your fish is being shipped to. Then decide if you need to try to keep the box hot or cold with heating pads or ice packs. Fast the fish a day before shipping would be best to make sure the water stays as clean as possible. I would recommend using bubble wrap to pack the box.